BROOK, CREEK, AND RIVER 



281 



segment of the antennae is nearly round in the former, very 

 elongate in the latter. The rest are included in four families 

 of the dragon flies. The Aeschnidac and Gomphidae have a flat 

 mask that does not cover the face. The former have six- or 

 seven-jointed slender antennae; the latter, four-jointed stout 



Fig. 426. — Nymphs of the dragon flies: a, Anax; b, Aeschna; c, Boyeria. 

 After Needham. 



antennae. The Cordulegasteridae and the Lihellulidae have a 

 mask that is spoon-shaped and that covers the face. The teeth 

 On its opposing edges in the former are large, acute, and inter- 

 locking; in the latter they are rounded. Family Aeschnidae 



includes of those mentioned above, 

 Anax, Aeschna, and Boyeria, the 

 nymphs of which have respectively 

 three, four, and five pairs of lateral 

 spines on the sides of the abdomen 

 Fig. 427.— Head of horned (Fig. 426). Family GompJiidae includes 



da.ce, Semotilus atromaculatiis. n ,7 t^- i . tt 



Progomplius, Diastatomma, Hagemus, 

 Dromogomphus, and Gomphiis. In ProgompJius the middle pair 

 of legs are set closer to each other than are the forelegs, in the 

 others at least as far apart as the forelegs, in Hagenius isLvthei 

 apart. Hagenius also has more than four pair of lateral spines, 

 the others not over four pair. The tenth or last abdominal 

 segment of Diastatomma narrows posteriorly, while that of 



